Former Lance Cpl. Kesaun Sykes, 27, is the fourth and final former Marine convicted of killing Sgt. Jan Pietrzak, 24, and his wife Quiana Jenkins-Pietrzak, 26, at their home in October 2008

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Updated at 6:51 AM CDT on Wednesday, Aug 6, 2014

A former Marine known to his comrades by the nickname “Psycho” was found guilty Tuesday in the 2008 robbery, torture and execution-style slayings of a fellow Marine and his wife in Southern California.

Sykes, an ex-lance corporal, is the fourth and final suspect to be found guilty in the case. The three others – also former Marines – were sentenced for the brutal murders in June 2013.

The jury also found Sykes guilty of special circumstances of murder during the commission of a robbery, burglary and rape by an instrument, the Riverside County District Attorney’s office confirmed. The jury deliberated less than two hours before reaching the guilty verdict.

Sykes faces the death penalty or life in prison without the possibility of parole for his crimes. The penalty phase of his trial begins Aug. 11 in which jurors will hear testimony to decide Sykes’ fate.

In October 2008, the Pietrzaks were found gagged, tied and shot in their home near Murrieta, Calif.

Sgt. Pietrzak, a helicopter airframe mechanic at MCAS Miramar near San Diego, was found bloodied and beaten to death. His wife's body was discovered naked. Investigators said she had been sexually assaulted before the couple was murdered.

Both victims were also found bound and shot in the head.

Investigators said racial slurs were found spray-painted throughout the Pietrzak’s home and fires had been set in an attempt to destroy evidence.

Prosecutors said robbery was the motive behind the murders. The four suspects went to the Pietrzak home with the intention to steal and forced their way inside.

Jewelry, including the couple’s wedding rings, as well as Pietrzak's dress uniform were found at the suspects' homes, authorities said.

John, Miller and Cox all worked with Sgt. Pietrzak at one time as Marines while stationed at Camp Pendleton.

John and Miller were both sentenced to the death penalty, while Cox was sentenced to life in prison without the possibility of parole.

Sgt. Pietrzak, who was born in Poland and raised in Brooklyn, N.Y., joined the Marines in 2003 and served in Iraq from July 2005 to February 2006. Relatives of the victims said Quiana was from San Bernardino and was a 2005 graduate of San Diego State University.

The couple met in San Diego through a mutual friend who also attended SDSU. Quiana was studying to become a doctor.